a) Field of the invention
The present invention relates to a bone polypeptide and active fragments thereof. In particular, the present invention relates to bone polypeptide-1, active fragments thereof and nucleic acids encoding the same.
b) Brief Description of the Prior Art
Bone is a specialized connective tissue that is constantly remodelled by the reciprocal action of bone-forming cells (osteoblasts) and bone-resorbing cells (osteoclasts) (Manolagas, 2000). Full citations for the references cited herein are provided before the claims. In the developing organism, the skeletal system is formed either through endochondral or intramembranous ossification (Baron, 1999). Endochondral bone formation, exemplified by the development of the long bones of the limbs, involves replacement of a preformed cartilage template whereas intramembranous ossification, a process typical of flat bones of the skull, does not rely on an intermediate cartilaginous step. In both cases, osteoblasts originate from the mesenchyme and initially differentiate from the inner layer of the periosteum. Periosteal osteoblasts deposit lamellar bone, progressively becoming enthumbed in the matrix they deposit and finally differentiating into osteocytes. Osteocytes elaborate cytoplasmic extensions through canaliculi, thereby constituting a network of interconnected cells within bone tissue (Aarder et al., 1994). It is thought that this network senses load on the bones and alters bone activity according to the demands of that load.
Osteoporosis is characterized by a loss of bone mass due to an imbalance between bone resorption and bone formation. This degenerative disease affects 20 million women in the United States. Current treatment mainly involves the inhibition of the activity of osteoclasts by inhibitors such as bisphosphonates (Russell, 1999). Such anti-resorptive therapies slow down the progression of the disease but do not really help in rebuilding lost bone. Effective bone anabolics are thus needed. Unfortunately, with the possible exception of a fragment of parathyroid hormone (PTH1-34; Neer et al., 2001), very few molecules have been shown to notably increase bone mass in osteoporotic patients. A number of growth factors and related molecules are now being considered as therapeutic agents. Insulin-like growth factor 1, (IGF1) is a protein that displays bone-sparing activity in the ovariectomized rat, a model of postmenopausal osteoporosis (Bagi et al., 1995). Basic fibroblast growth factor (bFGF) is another protein that was shown to enhance bone formation in vivo (Mayahara et al., 1993). However IGF1, as the name implies, is also an hypoglycemic factor and bFGF severely disrupts hematopoiesis. Hence, the in vivo specificity of these growth factors is difficult to ascertain and their clinical potential is unproven. There is thus a need to identify molecules that can efficiently and specifically increase bone formation.
Regulation of bone mass by the central nervous system has recently been reported by Ducy and colleagues (Ducy et al., 2000). These authors have shown that the ob/ob mice lacking a functional leptin gene have a higher bone mass. Leptin is a 16 kD hormone synthesized by the adipocytes and acting on hypothalamic neurons to regulate caloric intake (Unger, 2000). When injected intracerebroventricularly, leptin caused a loss of bone. It has been hypothesized that, in addition to adipocytes, osteoblasts per se might secrete a humoral factor that regulates bone mass through an hypothalamic relay. Thus, identification of such a factor might be useful to devise new therapies for the treatment of osteoporosis.
The inorganic component of bone is made up of hydroxyapatite crystals. Hence, in addition to its role as a supporting mechanical structure, the skeleton is a reservoir of calcium. Bone cells play a crucial role in the homeostasis of calcium and other ions such as phosphate. In particular, it has been proposed that bone cells secrete a hormone, tentatively called phosphatonin, that regulates phosphate retention by kidney tubules. It has been reported that phosphatonin levels are elevated in conditioned medium of tumor cells derived from a rare disease called oncogenic hypophosphatemia osteomalacia (Kumar, 2000). Furthermore, it has been postulated that phosphatonin is a substrate for Phex, a metallopeptidase found at the cell surface of osteoblasts and osteocytes (Frota Ruchon et al., 2000). This hypothesis is supported by the fact that patients with X-linked hypophosphatemia harbor deleterious mutations in the Phex coding sequence (The Hyp consortium, 1995) and presumably have elevated levels of active phosphatonin. Low serum phosphate levels are associated with impaired bone quality. Hypophosphatemia could be remedied by injection of phosphatonin antagonists. In view of the above, it is clear that there is a need to identify the molecular nature of phosphatonin or other molecules that regulate phosphate metabolism. In particular, there is a tremendous need to identify bone polypeptides that may be useful as therapeutic agents.